Have you tried the "Software Sources" GUI app? I'm not running 12.04, but in the past, this is where you could choose the repository location, and edit which ones to use from that location.
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Marty FriedApr 21 '12 at 18:14

I did, and none of the errors listed were in that. I just want to find a copy of the sources.list file as it was when I had freshly installed Ubuntu.
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KlikiniApr 21 '12 at 18:15

You should be able to go to the "Other Software" tab, and delete the duplicate entries. Or, if you're not sure, you can uncheck a few and see if that fixes the problem. Also, did you run "sudo apt-get update" like it suggested?
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Marty FriedApr 21 '12 at 18:34

Yes, I did. It gives me W: Failed to fetch http://ppa.launchpad.net/pmcenery/ppa/ubuntu/dists/precise/main/source/Sources 404 Not Found W: Failed to fetch http://ppa.launchpad.net/pmcenery/ppa/ubuntu/dists/precise/main/binary-amd64/Pac‌​kages 404 Not Found W: Failed to fetch http://ppa.launchpad.net/pmcenery/ppa/ubuntu/dists/precise/main/binary-i386/Pack‌​ages 404 Not Found E: Some index files failed to download. They have been ignored, or old ones used instead.
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KlikiniApr 21 '12 at 19:09

Somewhere in the past, you added ppa.launchpad.net/pmcenery/ppa/ubuntu/dists/precise/main/source/… to your list. Did you last have the natty version before the beta? I suspect the upgrader changed natty to precise, but there is no precise version available for this package. I would delete it from "Other Software". The error isn't hurting anything, by the way.
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Marty FriedApr 21 '12 at 20:27

2

Can you please put your new information in your question instead of the comments, it's hard to see what part you're having a problem you're stuck on now.
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Jorge CastroApr 21 '12 at 21:20